Form follows function

A pithy way of expressing the idea that the shape of a structure is determined at least in part by the function it performs. This is a useful idea to keep in mind when trying to understand why something (often a plant or animal) looks the way it does, or when you're designing something yourself.

If x is going to do y, then x had better be built in a way that allows y. This is a big idea in biology, where there is no reason for things to be all fancied up (except for functional reasons like sexual selection). As far as human-made devices go, a product's main function is usually to be sold, whatever the advertisements might say. You see some weird forms out there.